1. Technical Field
The present invention relates to an image forming apparatus such as an electro-photographic copying machine or printer, in particular to an image forming apparatus equipped with a coating brush for coating a rotating member with a lubricant.
2. Description of Related Art
The image forming apparatus such as an electron-photographic copying machine or printer uses various rotating members such as an image carrying member typical of which is a photographic sensitive drum, a transfer roller used in the secondary transfer position, or a belt or roller used in the fixing unit. It is performed widely to coat these rotating members with lubricants for various purposes, such as for improving the toner image transfer performance, the toner image fixing performance, the residual toner cleaning performance, and the abrasion resistance performance of the rotating member.
Solid salts of fatty acids such as zinc stearate are often used as the lubricant. As a member for coating the rotating member with the lubricant, a coating brush TB comprising a brush textile 464, which is a belt-like base material implanted with brush fabrics wrapped around a metal core 463, such as shown in FIG. 1, is used. In coating the rotating member with the lubricant, the coating brush TB provided to contact both the rotating member and the lubricant is rotated so that the rotating member is coated with a certain amount of the lubricant scraped up from the solid lubricant.
It is known that, in coating the rotating member with the lubricant from the coating brush TB, the coating amount of the lubricant can be controlled by the rotation of the coating brush TB. More specifically, the rotating speed of the coating brush TB is controlled according to a predetermined standard, examples of which can be seen in three documents, i.e., Patent Document 1 (Unexamined Japanese Patent Publication No. 2010-210799), Patent Document 2 (Unexamined Japanese Patent Publication No. 2010-169937), and Patent Document 3 (Unexamined Japanese Patent Publication No. 2007-225847). Patent Document 1 discloses controlling the coating weight by detecting the torque of the coating brush and changing the rotating speed of the coating brush corresponding to the obtained torque value in order to coat the photographic sensitive drum with the lubricant in a stable manner continuously. Patent Document 2 discloses controlling the rotating speed of the coating brush TB corresponding to the image area ratio, which is one of the printing conditions. Patent Document 3 discloses controlling the rotating speed of the coating brush TB corresponding to the consumption amount of the lubricant.
It is also known to control the rotating speed of the rotating member. This is because the rotating member has to be positioned in contact with the coating brush TB and the rotating speed of the rotating member may cyclically change due to the jouncing of the driving unit of the coating brush TB and the wobbling of the metal core 463 of the coating brush TB. Consequently, in certain models of image forming apparatus, the variation of the rotating speed of a rotating member within one revolution is detected using an encoder and the detected result is fed back to the subsequent control of the rotating speed of the rotating member.
In case of such an image forming apparatus, it is necessary to choose the ratio between the rotating speed of the rotating member and the rotating speed of the coating brush TB to be a multiple of a natural number in order to feed back the variation of the rotating speed of a rotating member within one revolution to the subsequent control of the rotating speed of the rotating member. For example, in order to arrange the rotating speed of a rotating member be 95.4 rpm and the rotating speed of a coating brush TB be 381.6 rpm, the ratio between the rotating speed of the rotating member and the rotating speed of the coating brush TB is chosen to be 1:4. As a result, it is inevitable that the surface of the rotating member contacts with the coating brush TB always at the same position.
As the brush textile 464 is spirally wrapped around the metal core 463, the density of the brush textile 464 varies along the longitudinal direction of the coating brush TB. Such a variation in the density becomes one of the causes of the unevenness of coating. More specifically, the coating amount of the lubricant to the rotating member is less in an area where the brush textile 464 is thin, while the coating amount of the lubricant to the rotating member is more in an area where the brush textile 464 is dense.
Consequently, if such a coating brush TB is used while maintaining the ratio between the rotating speed of the rotating member and the rotating speed of the coating brush TB at a multiple of a natural number, the unevenness of coating on the rotating member results because the surface of the rotating member contacts with the coating brush TB always at the same position. An example of such unevenness of coating is shown in FIG. 2. FIG. 2 is an expansion plan of the rotating member 41. The area A in the expansion plan is where the coating amount of lubricant decreases because the contact with the thin area of the brush textile 464 is made. The area B is where the coating amount of lubricant increases because the contact with the thick area of the brush textile 464 is made. Thus, the density variation of the brush textile 464 causes the unevenness of coating on the rotating member 41.
Therefore, it is customary to choose the ratio between the rotating speed of the rotating member and the rotating speed of the coating brush TB to stay away from a multiple of a natural number within a range of being able to feed back the variation of the rotating speed of a rotating member within one revolution normally to the subsequent control of the rotating speed of the rotating member. Specifically, it is controlled to make the rotating speed of the coating brush TB either slightly faster or slightly slower than the standard speed, which is a multiple of a natural number of the rotating speed of the rotating member. This makes it possible to avoid the surface of the rotating member from contacting with the coating brush TB always at the same position. However, such a countermeasure is not quite sufficient to suppress the unevenness of coating.